Underground drilling involves drilling a borehole through a formation deep in the Earth using a drill bit connected to a drill string. Two common drilling methods, often used within the same hole, include rotary drilling and slide drilling. Rotary drilling typically includes rotating the drilling string, including the drill bit at the end of the drill string, and driving it forward through subterranean formations. This rotation often occurs via a top drive or other rotary drive equipment at the surface, and as such, the entire drill string rotates to drive the bit. This is often used during straight runs, where the objective is to advance the bit in a substantially straight direction through the formation.
Slide drilling is often used to steer the drill bit to effect a turn in the drilling path. For example, slide drilling may employ a drilling motor with a bent housing incorporated into the bottom-hole assembly (BHA) of the drill string. During typical slide drilling, the drill string is not rotated and the drill bit is rotated exclusively by the drilling motor. The bent housing steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling. Alternatively, the steerable system can be operated in a rotating mode in which the drill string is rotated while the drilling motor is running.
Directional drilling can also be accomplished using rotary steerable systems which include a drilling motor that forms part of the BHA, as well as some type of steering device, such as extendable and retractable arms that apply lateral forces along a borehole wall to gradually effect a turn. In contrast to steerable motors, rotary steerable systems permit directional drilling to be conducted while the drill string is rotating. As the drill string rotates, frictional forces are reduced and more bit weight is typically available for drilling. Hence, a rotary steerable system can usually achieve a higher rate of penetration during directional drilling relative to a steerable motor, since the combined torque and power of the drill string rotation and the downhole motor are applied to the bit.
A problem with conventional slide drilling arises when the drill string is not rotated because much of the weight on the bit applied at the surface is countered by the friction of the drill pipe on the walls of the wellbore. This becomes particularly pronounced during long lengths of a horizontally drilled bore hole.
To reduce wellbore friction during slide drilling, a top drive may be used to oscillate or rotationally rock the drill string during slide drilling to reduce drag of the drill string in the wellbore. This oscillation can reduce friction in the borehole. However, too much oscillation can disrupt the direction of the drill bit and send it off-course during the slide drilling process, and too little oscillation can minimize the benefits of the friction reduction, resulting in low weight-on-bit and overly slow and inefficient slide drilling.
The parameters relating to the top-drive oscillation, such as the number of oscillating rotations, are typically programmed into the top drive system by an operator, and may not be optimal for every drilling situation. For example, the same number of oscillation revolutions may be used regardless of whether the drill string is relatively long or relatively short, and regardless of the sub-geological structure. Drilling operators, concerned about turning the bit off-course during an oscillation procedure, may under-utilize the oscillation features, limiting its effectiveness. Because of this, in some instances, an optimal oscillation may not be achieved, resulting in relatively less efficient drilling and potentially less bit progression.
As such, drilling may be controlled through improved steering control systems. The steering control systems may provide steering corrections using reactive steering that may provide instructions based on toolface position and proactive steering based on differential pressure changes. Such steering corrections may be made by adjusting and/or offsetting a quill position of the drilling apparatus. However, under certain conditions, steering with quill position offsets may be ineffective under certain drilling conditions. Accordingly, improved automated steering control is needed.